Effects of Dam Regulation on the Hydrological Alteration And

water

Article Eﬀects of Dam Regulation on the Hydrological Alteration and Morphological Evolution of the Volta River Delta

Mawusi Amenuvor 1, Weilun Gao 1, Dongxue Li 1 and Dongdong Shao 1,2,* 1 State Key Laboratory of Water Environment Simulation & School of Environment, Beijing Normal University, Beijing 100875, China; m.amenuvor_kﬁ@yahoo.com (M.A.); [email protected] (W.G.); [email protected] (D.L.) 2 Tang Scholar, Beijing Normal University, Beijing 100875, China * Correspondence: [email protected]; Tel.: +86-10-5880-5053

Received: 16 January 2020; Accepted: 27 February 2020; Published: 28 February 2020

Abstract: The Volta River in West Africa is one of the most regulated rivers influenced by dams in the world, and the regulation has resulted in substantial impacts on the hydrological alteration and morphological evolution of the Volta River Delta. However, comprehensive analyses of the relevant effects are still lacking to date. In this study, inter-annual variations of river discharge and sediment load for pre- and post-Akosombo Dam periods (1936 to 2018) were analyzed through simple regression and Mann–Kendall (MK) trend analysis whereas the intra-annual variations were dictated by the non-uniformity and regulated coefficients. The shoreline changes were further evaluated using Landsat remote sensing images (1972 to 2018) to explore the effects of hydrological alteration on the morphological evolution of the Volta River Delta. Hydrological analyses show that the inter- and intra-annual variations are much higher in the pre-dam period, suggesting the substantial regulation of the Akosombo Dam on the Volta River. The dam regulation has more significant effects on the sediment load delivered to the delta than the river discharge, which decreased by 92.32% and 23.23%, respectively. Morphological analyses show that the progradation-erosion of the Volta River Delta constantly fluctuates within a relatively small range (maximum 0.5%) after the 1970s. The relationship between the variations of the delta area and sediment load implicates that a quasi-equilibrium state may have been established at the Volta River Delta, given the current sediment load. Our findings provide references for the future regulation and restoration of the Volta River Delta.

Keywords: dam regulation; hydrological alteration; morphological evolution; Volta River Delta

1. Introduction Deltas host more than half a billion of the world’s population and one of the most productive ecosystems on Earth [1,2]. However, the world’s deltas are subject to continuous climate change and anthropogenic activities, such as sea-level rise, subsidence, and reduced sediment load, etc. [1,3–6]. Among these natural and anthropogenic stressors, dam regulations that lead to hydrological alteration have caused substantial impacts on the morphological evolution of the world’s deltas [7–11]. Understanding the potential effects of dam regulations on the affected deltas’ hydrology and morphology thus becomes a vital task in their protection and restoration [12,13]. The Volta River in West Africa is considered one of the most regulated rivers by dams worldwide [8,14–16]. The Volta River Delta was richly supplied with sediments from the Volta River [17]. However, the construction of the Akosombo Dam in 1964 has intercepted and reduced significantly fluvial sediment supply from the river with a decreasing rate of 90% [17,18]. The drastically reduced sediment supply has caused severe adverse effects on the river delta [19], e.g., coastal erosion [20]. Furthermore,

Water 2020, 12, 646; doi:10.3390/w12030646 www.mdpi.com/journal/water Water 2020, 7, x FOR PEER REVIEW 2of12 Water 2020, 12, 646 2 of 12 drastically reduced sediment supply has caused severe adverse effects on the river delta [19], e.g., thecoastal reduction erosion in [ fluvial20]. Furthermore, sediment supply the reduction to the Volta in fluvial River Delta sediment has resulted supply into thethe regimeVolta R shiftiver ofDelta the Voltahas resulted River Delta in the from regime the river-dominated shift of the Volta delta River to wave-dominatedDelta from the river delta-dominated [20,21]. In additiondelta to wave to the- reductiondominated of delta the fluvial [20,21 sediment]. In addition input, theto the high reduction alongshore of sediment the fluvial transport sediment has also input exacerbated, the high thealongshore erosion ofsediment the delta transport [20]. has also exacerbated the erosion of the delta [20]. Considering the the extent extent of damof dam regulations regulations on the on Volta the River Volta DeltaRiver and Delta the economic and the economic and ecological and importanceecological ofimportance the area [20 of,22 ], the significant area [20 attention,22], significant has been given attention to the has delta been [17,18 given,20,23 ,24 to]. the However, delta apart[17,18 from,20,23 Ly,24 (1980)]. However, [18] who apart quantitatively from Ly (1980) analyzed [18 the] w sedimentho quantitatively retention analyzed by the Akosombo the sediment Dam andretention its impacts by the on Akosombo the shoreline Dam of and the its delta impact severals on decades the shoreline ago, little of the research delta several has been decades conducted ago, tolittle quantify research the has subsequent been conducted influences to quantify of the dam the subsequent regulations influence on the hydrologicals of the dam alterations regulations and on morphologicalthe hydrological evolution alterations of theand Volta morphological River Delta. evolution of the Volta River Delta. InIn this this study, study, the the hydrological hydrological alterations alterations were wereanalyzed analyzed using monthly using monthly and annual and river annual discharge river anddischarge sediment and load sediment from load1936 fromto 2018. 1936 The to 2018inter-annual. The inter variations-annual variationsfor pre- and for post-Akosombo pre- and post- DamAkosombo periods Dam were periods analyzed were using analyzed simple using regression simple andregression the Mann–Kendall and the Mann (MK)–Kendall trend (MK) analysis trend in Sectionanalysis 3.1 in .Sec Thetion intra-annual 3.1. The intra variations-annual variations represented represented by the non-uniformity by the non-uniformity coefficient coefficient (Cy) and ( theCy) r regulatedand the regulated coefficient coefficient (Cr) were (C calculated) were calcula for theted pre-Akosombo for the pre-Akosombo (1936–1964), (1936 the–1964), pre-Kpong the pre (1970-Kpong to 1982)(1970 and to 1982)the post-Kpong and the post (1983–2018)-Kpong Dam (1983 periods–2018) toDam examine periods the toimpacts examine of dam the regulation impacts of during dam diregulationfferent periods during in different Section periods3.2. Available in Section Landsat 3.2. A remotevailable sensing Landsat images remote of sensing the Volta images River of Delta the fromVolta 1972River to 2018Delta were from interpreted 1972 to 2018 to analyze were interpreted the associated to morphologicalanalyze the associated evolution morphological in Section 3.3. Theevolution paper in is concludedSection 3.3. with The apaper summary is concluded of the major with findingsa summary of the of the study. major findings of the study.

2. Methodology

2.1. Study SiteSite

The VoltaVolta RiverRiver DeltaDelta isis thethe coastalcoastal plainplain ofof thethe VoltaVolta River,River, which which ranges ranges from from 5 5°25◦250' toto 66°20◦200' NN 2 and 00°4◦40' to 11°10◦10'0 E along the eastern coast of Ghana with aa totaltotal areaarea ofof approximatelyapproximately 45624562 kmkm2 inin 2 Ghana [22]] andand anan activeactive portionportion nearnear thethe riverriver mouthmouth ofof approximatelyapproximately 24302430 kmkm2 [[2525],], formingforming thethe interfaceinterface betweenbetween thethe VoltaVolta RiverRiver andand thethe AtlanticAtlantic OceanOcean (Figure(Figure1 1).). As As one one of of the the largest largest riverriver deltasdeltas inin WestWest Africa,Africa, thethe VoltaVolta RiverRiver DeltaDelta isis aa sanctuarysanctuary forfor aboutabout 80%80% ofof migratorymigratory birdsbirds thatthat stop-overstop-over inin Ghana,Ghana, andand hostshosts importantimportant speciesspecies inin itsits lagoonlagoon andand mangrovemangrove habitatshabitats [[2020].]. InIn additionaddition toto itsits ecologicalecological functions,functions, thethe delta is also of crucial socio-economicsocio-economic importance [26]] with diverse economic activities comprisingcomprising agriculture,agriculture, ﬁshery,fishery, saltsalt harvesting,harvesting, and and tourismtourism [ 22[22].]. The Akosombo and Kpong D Damsams were built on the Volta Volta River in 1964 and 1982, 1982, respectively. respectively. TheThe twotwo dams dams generate generate about about 1060 1060 MW MW of hydropower,of hydropower providing, providing 80% of80% Ghana’s of Ghana electric’s electric power power needs. Theneeds. Akosombo The Akosombo Dam is D aboutam is 100 about km 10 upstream0 km upstream from the fro riverm the mouth riveralong mouth the along river the channel, river channel and the, Kpongand the Dam Kpong is 24 D kmam downstream is 24 km downstream of Akosombo of alongAkosombo the river along channel. the river The channel reservoir. The capacity reservoir and areacapacity forthe and Akosombo area for the Dam Akosombo are 147,960 Dam million are 147,960 m3 and milli 8482.250on m3 and km 82482, whereas.250 km those2, whereas for the those Kpong for Damthe Kpong are 200 Dam million are 200 m3 andmillion 25.2 m km3 and2. 25.2 km2.

Figure 1. The location of the Volta River Delta. Figure 1. The location of the Volta River Delta.

2.2. Datasets

Water 2020, 12, 646 3 of 12

2.2. Datasets Annual river discharge data of the Volta River (1936 to 1961) downstream of the Akosombo Dam at the Senchi gauge station (Figure1) were provided by the Water Resources Institute of the Council for Scientiﬁc and Industrial Research (CSIR-WRI), Ghana. Monthly river discharge data downstream of Akosombo Dam (1970 to 2018) and Kpong (1984 to 2018) dam at the Akosombo and Kpong gauge station (Figure1), respectively, were acquired from the Volta River Authority, Ghana. The average monthly river discharge (1936 to 1961) before the construction of the Akosombo Dam was obtained from CSIR-WRI, Ghana. We further calculated monthly ratios from the average monthly river discharge data for the pre-dam period and then derived the monthly river discharge data for the pre-dam period by dividing the annual discharge according to the ratios of the river discharge for diﬀerent months. As directly measured sediment data on the Volta River was unavailable, the sediment rating curve [17] was used to estimate the sediment load from the associated river discharge (details elaborated in Section 2.3). High-quality Landsat remote sensing data of the Volta River Delta coastline from a Multispectral Scanner (MSS), a Thematic Mapper (TM), and an Enhanced Thematic Mapper (ETM+) were downloaded from USGS (https://earthexplorer.usgs.gov). Two separate sets of remote sensing images that cover the whole Volta River Delta, i.e., path 192, row 56 and path 193, row 56 for TM and ETM images (or path 206, row 56 and path 207, row 56 for MSS images), with least possible time interval as well as minimum cloud cover along the shoreline were selected for each year. Table1 shows relevant information on the acquisition dates, image type, resolution, and band type of remote sensing images. Notably, the two dates in each acquisition date cell in Table1 are for path 192, row 56 and path 193, row 56 for TM and ETM images (or path 206, row 56 and path 207, row 56 for MSS images), respectively.

Table 1. List of Landsat images used in this study.

Image Resolution Image Resolution Acquisition Date Band Acquisition Date Band Type (m) Type (m) 10 November 1972/ 16 November 2005/ 9 November 1972 25 November 2005 28 December 1975/ 21 December 2006/ 3 December 1975 30 December 2006 MSS 60 4 20 December 1978/ 24 December 2007/ 3 December 1978 1 December 2007 10 December 1979/ 10 December 2008/ 10 December 1979 19 December 2008 11 October 1984/ 27 November 2009/ 23 November 1984 22 December 2009 6 April 1985/ 16 December 2010/ 16 April 1985 25 December 2010 22 December 1986/ TM 30 6 19 December 2011/ 31 December 1986 28 December 2011 ETM+ 30 6 5 October 1987/ 21 December 2011/ 30 October 1987 30 December 2012 20 November 1998/ 24 December 2013/ 30 November 1998 17 December 2013 16 November 1999/ 27 December 2014/ 9 November 1999 4 December 2014 13 December 2000/ 14 December 2015/ 4 December 2000 23 December 2015 6 December 2001/ 16 December 2016/ 14 November 2001 ETM+ 30 6 9 December 2016 6 December 2002/ 3 December 2017/ 17 November 2002 26 November 2017 26 December 2003/ 22 December 2018/ 20 November 2003 31 December 2018 31 December 2004/ 24 December 2004 Water 2020, 12, 646 4 of 12

2.3. Estimation of Sediment Load The relationship between sediment load and river discharge on major coastal rivers in Ghana proposed by Boateng et al. (2012) [17] was adopted in this study to estimate the sediment load, which reads: Qs = 49.7Qw + 20.98 (1)

3 3 where Qw represents the river discharge (m /s), and Qs denotes the suspended sediment load (m /day). The estimates of sediment load before Akosombo Dam construction were multiplied by 10 to reflect the reported about 90% reduction of sediment post-Akosombo Dam in the literature such as [17,18,27]. The assumption is also consistent with the sediment trapping efficiency of the Akosombo Dam reported in reference [28], which ranges from 87% to 96% for different calculation formulas. Moreover, the annual sediment load estimated from the sediment rating curve was around 180 million m3/year before dam construction, which was close to the reported estimated yield prior to dam construction (153 million m3/year) in reference [29]. The post-dam estimates of about 10 to 13 million m3/year also match the measured sediment load after dam construction (12 million m3/year) in reference [17]. All data mentioned above refer to the sediment load delivered to the river delta. In this study, we focused on the effects of sediment trapping and alteration of river discharge induced by dams on the sediment load by using a sediment rating curve. The effects of the land use/land cover changes and climate change (such as rainfall) on the sediment yield were neglected since the sediment trapping efficiency of the Akosombo Dam has been proved to be the dominant factor for the changes of sediment load downstream the dam [28]. Notably, however, the sediment load calculated in this study was derived from the river discharge at the dam and thus should be considered a proxy of sediment load to the delta. In reality, the riverbed downstream of the dam could make a morphological adjustment to the altered flow regime, and hence ameliorate the reduced sediment load to the delta caused by dams.

2.4. Hydrological Alteration The mean and standard deviations of annual river discharge and sediment load were calculated for the period before Akosombo Dam construction (1936 to 1963) and after Akosombo Dam construction (1970 to 2018) to explore the relationship between dam regulation and inter-annual variations of river discharge and sediment load. Mann–Kendall (MK) trend test was employed to detect the trend and test its signiﬁcance. The method tests an assumption of the beginning of a developing trend within a sample (xi, xj, ... , xn) of the random variable X, based on the rank series R of the progressive or retrogressive rows of this sample [30]. The MK test statistic, dk is calculated as [31,32]:

Xk d = R (2 k n) (2) k i ≤ ≤ i=1 ( +1 x > x R = i j (j = 1, 2, ... i) (3) i 0 otherwise

Under the null hypothesis of no trend, the statistic dk is distributed as a normal distribution with the expected value of E[dk] and the variance Var[dk] as follows [31,32]:

n(n 1) E[d ] = − (4) k 4

n(n 1)(2n + 5) Var[d ] = − (5) k 72

dk E[dk] Zk = p − (k = 1, 2, 3 ... , n) (6) Var[dk] Water 2020, 12, 646 5 of 12

The Zk values calculated with progressive and retrograde series of X are denoted as UFk and UBk, respectively. A positive or negative UFk value represents an increasing or decreasing trend. When |UFk| < 1.96, the trend is not significant within a 95% confidence level. The intersecting points of the UFk and UBk lines indicate the beginning time of a developing trend of the time series. Nonuniformity coefficient, Cy, which compares the standard deviation with the mean discharge, and regulated coefficient, Cr, which represents the total monthly water/sediment discharges that are larger than the mean river discharge, from Li et al. (2012) [33] were adopted to quantify the intra-annual variation and distribution of river discharge and sediment load: σ Cy = (7) Q uv t 12 1 X 2 σ = (Q(t) Q) (8) 12 − t=1

X12 X12 Cr = ψ(t)[Q(t) Q]/ Q(t) (9) − t=1 t=1 ( 0 Q(t) < Q ψ(t) = (10) 1 Q(t) Q ≥ where σ is the standard deviation of the monthly river discharge, Q(t) is the monthly river discharge or sediment load for t-month, and Q is the average of Q(t). The coeﬃcients were compared for pre- and post-dam periods. Cy and Cr attain zero when the monthly river discharges are the same in a year, and larger Cy and Cr represent more variability of river discharge and sediment load.

2.5. Morphological Evolution In this study, the following steps were taken for remote sensing analyses. Firstly, the 2 separate sets of remote sensing images that cover the whole Volta River Delta were merged, from which the images of the study area were extracted using coordinates (6.3◦ N, 0.226◦ W) for the upper left and (5.2◦ N, 1.81◦ E) for lower right. The projection coordinates for the images were UTM 31 N. Secondly, the pre-processed images were used to detect the shoreline manually using 432 bands and 543 bands pseudo-color image for MSS and TM/ETM+ images, respectively [13,34], which allowed the accurate identiﬁcation and delineation of the shoreline (Figure2a). In order to ensure consistency and accuracy on the derived delta area, a common polygon (white rectangle in Figure2b) was used to intersect with the delineated shoreline (red line in Figure2b). The coordinates selected for the polygon (in UTM 31 N) were (146,679, 588,419) for the lower left and (314,146, 684,940) for the upper right. Finally, the delta area within the polygon and the delineated shoreline were calculated (Figure2b). The tidal range of the Volta Delta was about 1 m, which was relatively small. As such, the uncertainty of the shoreline extraction due to tidal variation was presumably small [23]. To assess the delta areal changes, the areal changes at the end of two consecutive years were calculated. A positive value represents progradation, and a negative value represents erosion. To compare the inﬂuence of sediment load on the delta areal changes, the corresponding cumulative sediment load for each period was calculated and then plotted against the associated delta areal changes following [35]. Water 2020, 7, x FOR PEER REVIEW 5of12

|UFk| < 1.96, the trend is not significant within a 95% confidence level. The intersecting points of the UFk and UBk lines indicate the beginning time of a developing trend of the time series. Nonuniformity coefficient, Cy, which compares the standard deviation with the mean discharge, and regulated coefficient, Cr, which represents the total monthly water/sediment discharges that are larger than the mean river discharge, from Li et al. (2012) [33] were adopted to quantify the intra- annual variation and distribution of river discharge and sediment load:  C  (7) y Q

1 12  2   (())Q t Q (8) 12 t 1

12 12 C ( t )[ Q ( t ) Q ]/ Q ( t ) r tt11 (9)

0 Q ( t )  Q  ()t   (10) 1 Q ( t )  Q where  is the standard deviation of the monthly river discharge, Q(t) is the monthly river discharge or sediment load for t-month, and 푄̅ is the average of Q(t). The coefficients were compared for pre- and post-dam periods. Cy and Cr attain zero when the monthly river discharges are the same in a year, and larger Cy and Cr represent more variability of river discharge and sediment load.

2.5. Morphological Evolution In this study, the following steps were taken for remote sensing analyses. Firstly, the 2 separate sets of remote sensing images that cover the whole Volta River Delta were merged, from which the images of the study area were extracted using coordinates (6.3° N, 0.226° W) for the upper left and (5.2° N, 1.81° E) for lower right. The projection coordinates for the images were UTM 31 N. Secondly, the pre-processed images were used to detect the shoreline manually using 432 bands and 543 bands pseudo-color image for MSS and TM/ETM+ images, respectively [13,34], which allowed the accurate identification and delineation of the shoreline (Figure 2a). In order to ensure consistency and accuracy on the derived delta area, a common polygon (white rectangle in Figure 2b) was used to intersect with the delineated shoreline (red line in Figure 2b). The coordinates selected for the polygon (in UTM 31 N) were (146,679, 588,419) for the lower left and (314,146, 684,940) for the upper right. Finally, the delta area within the polygon and the delineated shoreline were calculated (Figure 2b). The tidal range of the Volta Delta was about 1 m, which was relatively small. As such, the uncertainty of the shoreline extraction due to tidal variation was presumably small [23]. To assess the delta areal changes, the areal changes at the end of two consecutive years were calculated. A positive value represents progradation, and a negative value represents erosion. To compare the influence of sediment load on the delta areal changes, the corresponding cumulative sedimentWater 2020 , load12, 646 for each period was calculated and then plotted against the associated delta area6 of 12l changes following [35]. Water 2020, 7, x FOR PEER REVIEW 6of12 (a) MSS (b) of the Volta River Delta. The red line is the example of extracted shoreline, and the white polygon is the intersected polygon. TM 3. Results and Discussion

3.1. Effects of DamETM Regulation on Inter-Annual Variations of River Discharge and Sediment Load The annual river discharge and sediment load for the period from 1936 to 1963 (pre-Akosombo Dam construction) and that from 1970 to 2018 (post-Akosombo Dam construction) were plotted in FigureFigure 2. 2. (a()a P) Pseudo-colorseudo-color images images using using 432 432 bands bands and and 543 543 bands bands for for the the multispectral multispectral scanner scanner (MSS (MSS)) Figure 3. The average river discharge and sediment load for the pre-dam period were 1237.00 m3/s andand thematic thematic mapper mapper (TM (TM))/enhanced/enhanced thematic thematic mapper mapper (ETM (ETM++) )images images (b (b) )extraction extraction of of the the shoreline shoreline and 6.87 m3/s with a standard variation of 516.80 m3/s and 2.87 m3/s, respectively, whereas those for of the Volta River Delta. The red line is the example of extracted shoreline, and the white polygon is the the post-dam period were 949.55 m3/s and 0.53 m3/s with a standard variation of 196.10 m3/s and 0.11 intersected polygon. m3/s, respectively. Decreasing percentages of river discharge and sediment loads were 23.23% and 3. Results92.32%, andrespectively, Discussion after the construction of the Akosombo Dam. The total decreasing rate of sediment load was subject to two concurrent effects, namely, the 3.1.decrease Eﬀects of of Dam river Regulation discharge on and Inter-Annual the sediment Variations trapping of River in the Discharge reservoir. and SedimentUsing the Load sediment rating curveThe given annual in riverEquation discharge (1) and and incorporating sediment load a 90% for sediment the period load from reduction 1936 to yields 1963 (pre-Akosombo a total decreasing Damrate construction) of 92.32%. If and we that only from consider 1970 toed 2018 the (post-Akosombo reduction of sediment Dam construction) load due to were decreasing plotted river in Figuredischarge,3. The averagethe decreasing river discharge rate will and drop sediment to 23.23% load. for the pre-dam period were 1237.00 m 3/s and 6.87 m3Notably,/s with a standardthe reduction variation of sediment of 516.80 load m3 /dues and to 2.87 dam m retention3/s, respectively, can result whereas in the those erosion for of the the post-damdownstream period river were channel, 949.55 m which3/s and could 0.53 moffset3/s with the asediment standard reduction variation ofto 196.10the river m3 /deltas and. 0.11In addition, m3/s, respectively.the reduction Decreasing of flood peaks percentages caused of by river the dam discharge could and reduce sediment the sediment loads were carrying 23.23% capacity, and 92.32%, causing respectively,the opposite after effect. the constructionHowever, the of investigation the Akosombo of the Dam. above-mentioned effects needs [36,37] detailed morphodynamic modeling of the river channel downstream of the dam.

(a) (b) 3000 16 2750 Akosombo dam construction Akosombo dam construction 14 2500

/s)

/s) 3 2250 12

3 2000 10 1750 1500 8 1250 6 1000 750 4

Sediment load (m River discharge (m 500 2 250 0 0 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 Year Year Figure 3. (a) Annual river discharge and (b) annual sediment load of the Volta River before and after Figure 3. (a) Annual river discharge and (b) annual sediment load of the Volta River before and after Akosombo Dam construction. Akosombo Dam construction.

TheAs total shown decreasing in Figure rate4, the of MK sediment analyses load show wased that subject trends to of two river concurrent discharge and effects, sediment namely, load thebefore decrease the of construction river discharge of the and Akosombo the sediment Dam trapping shift between in the in reservoir.creasing and Using de thecreasing sediment trends ratingconstantly curve given with the in Equation UFk values (1) fluctuating and incorporating above and a 90% below sediment zero. loadHowever, reduction the trends yields we a totalre not decreasingsignificant rate under of 92.32%. the 95% If we confidence only considered level (within the reduction  1.96) of. However, sediment load there due wa tos a decreasing continuous riverdecreasing discharge, trend the decreasingafter 1970 (the rate inter willsec dropting to points 23.23%. of the UFk and UBk lines) for both river discharge andNotably, sediment the load reduction with consistent of sediment negative load dueUFk tovalues dam. retentionHowever, can the resultdecreasing in the trend erosion of the of theriver downstreamdischarge wa rivers not channel, significant which under could theoffset 95% the confidence sediment reduction level, while to the the river decreasing delta. In trend addition, of the thesediment reduction load of flood was significant peaks caused, which by the wa dams consistent could reduce with the larger sediment decreasing carrying rate capacity, of the causing sediment theload opposite mentioned effect. above. However, the investigation of the above-mentioned effects needs [36,37] detailed morphodynamic modeling of the river channel downstream of the dam. As shown in Figure4, the MK analyses showed that trends of river discharge and sediment load before the construction of the Akosombo Dam shift between increasing and decreasing trends

Water 2020, 12, 646 7 of 12

constantly with the UFk values fluctuating above and below zero. However, the trends were not significant under the 95% confidence level (within 1.96). However, there was a continuous decreasing ± trend after 1970 (the intersecting points of the UFk and UBk lines) for both river discharge and sediment load with consistent negative UFk values. However, the decreasing trend of the river discharge was not significant under the 95% confidence level, while the decreasing trend of the sediment load was Water 2020, 7, x FOR PEER REVIEW 7of12 significant, which was consistent with the larger decreasing rate of the sediment load mentioned above.

Figure 4. Mann–Kendall (MK) trend test analysis of annual: (a) River discharge and (b) sediment load Figure 4. Mann–Kendall (MK) trend test analysis of annual: (a) River discharge and (b) sediment load of the Volta River. of the Volta River. Generally, the construction of the Akosombo Dam plays an important role in the decreases of river Generally, the construction of the Akosombo Dam plays an important role in the decreases of discharge and sediment load to the Volta River Delta from 1963 to 2018 (Figures3 and4). Furthermore, river discharge and sediment load to the Volta River Delta from 1963 to 2018 (Figures 3 and 4). the dam appears to reduce the inter-annual changes of the annual river discharge and sediment load Furthermore, the dam appears to reduce the inter-annual changes of the annual river discharge and by regulating the natural variability of the flow regime of the Volta River, which is indicated by the less sediment load by regulating the natural variability of the flow regime of the Volta River, which is fluctuated UFk values above and below zero. indicated by the less fluctuated UFk values above and below zero. 3.2. Effects of Dam Regulation on the Intra-Annual Variations of River Discharge and Sediment Load 3.2. Effects of Dam Regulation on the Intra-Annual Variations of River Discharge and Sediment Load The nonuniformity coefficient (Cy) and regulated coefficient (Cr) were used to describe the intra-annualThe nonuniformity variations ofcoefficient river discharge (Cy) and and regulated sediment coefficient load in (C thisr) were study. used Table to describe2 shows the that intra the- annualconstruction variation of thes of dams river results discharge in a smaller and sedimentCy and C r load, which in suggests this study. a more Table uniform 2 show intra-annuals that the constructionriver discharge of the and dam sediments results load in a forsmaller the VoltaCy and River Cr, which Delta, suggests and the a Akosombo more uniform Dam intra affects-annual the riverintra-annual discharge variations and sediment of river load discharge for the Volta and sediment River Delta, load and more the significantly Akosombo D thanam affect the Kpongs the intra dam.- annualIn variation this study,s of riverriver dischargedischarge and and sediment sediment load load more exhibited significantly a substantial than the decrease Kpong dam. after the construction of the Akosombo Dam (Figure3), and the beginning of the decreasing trend coincides with the timeTable of 2. the The construction nonuniformity of thecoefficient Akosombo (Cy) and Dam regulated (Figure4 coefficient). Furthermore, (Cr) for thethe intra-annualriver discharge variations and sediment load for the periods of pre-Akosombo Dam, pre-Kpong Dam, and post-Kpong Dam. of river discharge and sediment load also exhibited a significant decease after the dam construction (Table2). Therefore, the Akosombo DamR wasiver presumably Discharge dominant in inducing the hydrological Coefficient Pre-Akosombo Pre-Kpong Post-Kpong

Cy 1.45 0.07 0.09 Cr 0.55 0.03 0.04 Sediment load Cy 1.44 0.04 0.09 Cr 0.55 0.03 0.04

In this study, river discharge and sediment load exhibited a substantial decrease after the construction of the Akosombo Dam (Figure 3), and the beginning of the decreasing trend coincides

Water 2020, 12, 646 8 of 12 alteration. Notably, however, the land use/land cover changes and climate change (such as rainfall) can also aﬀect the water and sediment yield from the catchment and cause additional eﬀects [38].

Table 2. The nonuniformity coefficient (Cy) and regulated coefficient (Cr) for the river discharge and sediment load for the periods of pre-Akosombo Dam, pre-Kpong Dam, and post-Kpong Dam. Water 2020, 7, x FOR PEER REVIEW 8of12 River Discharge Coefficient Pre-Akosombo Pre-Kpong Post-Kpong with the time of the construction of the Akosombo Dam (Figure 4). Furthermore, the intra-annual C 1.45 0.07 0.09 variations of river dischargey and sediment load also exhibited a significant decease after the dam Cr 0.55 0.03 0.04 construction (Table 2). Therefore, the AkosomboSediment Dam load was presumably dominant in inducing the hydrological alteration. NotablyCy , however,1.44 the land use/land 0.04 cover changes 0.09 and climate change (such as rainfall) can also affectC ther water and0.55 sediment yield 0.03 from the catchment 0.04 and cause additional effects [38]. 3.3. Effects of Dam Regulation on Delta Morphological Evolution 3.3. Effects of Dam Regulation on Delta Morphological Evolution Figure5 shows that the pseudo-color images using 432 bands and 543 bands for MSS and TM/ETMFigure+ images,5 shows respectively, that the pseudo separate-color the images land andusing ocean 432 band distinctly.s and The543 shorelinesbands for MSS (shown and in TM/ red ETMlines)+ extractedimages, respectively, from the Volta separate River the Delta land from and 1972 ocean to 2018distinctly. are also The shown shoreline in Figures (shown5, and in red the lines) delta extractedarea was furtherfrom the calculated Volta River as theDelta area from encompassed 1972 to 2018 by are the also shoreline shown and in Figure the landward 5, and the boundaries. delta area was further calculated as the area encompassed by the shoreline and the landward boundaries.

FigureFigure 5. 5. SShorelineshorelines extracted extracted from from the the remote remote sensing sensing images images from from 1972 to 2018.

As shown in Figure 6a, the calculated delta area ranges from 7064 km2 to 7107 km2. The percentages of progradation-erosion shown in Figure 6b suggests a stable delta area since the 1970s, despite the alternating progradation and erosion in different years. The highest progradation (0.56%) occurred in the period from 1975 to 1978. The minimum progradation-erosion was recorded at 0.01%. The alternating progradation and erosion patterns of the Volta River Delta were consistent with the field observations of the Appeaning Addo (2015) [23].

Water 2020, 12, 646 9 of 12

As shown in Figure6a, the calculated delta area ranges from 7064 km 2 to 7107 km2. The percentages of progradation-erosion shown in Figure6b suggests a stable delta area since the 1970s, despite the alternating progradation and erosion in diﬀerent years. The highest progradation (0.56%) occurred in the period from 1975 to 1978. The minimum progradation-erosion was recorded at 0.01%. The alternating progradation and erosion patterns of the Volta River Delta were consistent with the ﬁeld observations ofWater the 2020 Appeaning, 7, x FOR PEER Addo REVIEW (2015) [23]. 9of12

(a) 7120 7110

)

2 7100 7090 7080

Delta area (km area Delta 7070 7060 Year 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 (b) 50 40

)

2 30 20 10 0 -10 -20

Areal change (km change Areal -30 -40 -50 Year

1972-19751975-19781979-19841979-19841985-19851985-19861986-19871987-19981998-19991999-20002000-20012001-20022002-20032003-20042004-20052005-20062006-20072007-20082008-20092009-20102010-20112011-20122012-20132013-20142014-20152015-20162016-20172017-2018

Figure 6. Results of (a) delta area and (b) progradation-erosion patterns over different periods of the VoltaFigure River 6. Results Delta. of (a) delta area and (b) progradation-erosion patterns over different periods of the Volta River Delta. Previous studies showed that the Volta River Delta was subject to severe local erosion due to the significantPrevious reduction studies ofshowed sediment that load the Volta at an River earlier Delta stage was of the subject post-dam to severe period local [18 erosion]. In their due study,to the ansignificant average reduction shoreline of retreat sediment derived load from at an two earlier town stage maps of betweenthe post-dam 1923 peri andod 1949 [18 in]. In the their Keta study area, (Figurean average1, red shoreline circle) was retreat reported derived at 4 mfrom/year. two The town retreat map rates between increased 1923 to 6and m/year 1949 from in the 1956 Keta to 1975,area with(Figure the 1 most, red rapidcircle) erosion was reported occurring at 4 after m/ year 1964. atTh ane retreat average rate value increased of 8 to to 10 6 m m/year/ year [18 from]. However, 1956 to based1975, with on the the regional most rapid results erosion of our occurr study,ing the after shoreline 1964 at changes an average at the value latter of stage 8 to of10 them/ post-damyear [18]. periodHowever, (1972–2018) based on seemed the regional to reach results a quasi-equilibrium of our study, the state shoreline with minimal changes change. at the latter stage of the post-Thedam stable period shoreline (1972–2018) was consistentseemed to withreach some a quasi recent-equilibrium studies on state local with shorelines, minimal which change analyzed. the progradation-erosionThe stable shoreline wa patternss consistent of several with some sections recent of stu thed shorelineies on local along shorelines the Ghana, which coast analyzed [23]. Theirthe progradation results showed-erosion that patternsthe selected of several shoreline sections sections of ofthe the shoreline Volta River along Delta the Ghana were accreting coast [23 at]. anTheir average results rate showed of about that 0.53 the mselected/year, in shoreline contrast sections to the earlier of thetrend Volta reportedRiver Delta with were a reduction accreting of at the an shorelineaverage rate at a of rate about of about 0.53 8m/ to 10year m,/ yearin contrast [18]. The to progradationthe earlier trend of the report threeed sections with a wasreduction attributed of the to theshoreline implementation at a rate of of about the Keta 8 to 10 sea m/ defense year [18 project]. The [ 23progradation]. of the three sections was attributed to theThe impl correlationementation of of the the areal Keta changes sea defen ofs thee project Volta [ River23]. Delta with the associate sediment load for theThe period correlation from 1972 of the to area 2018l change (Figures7 of) showed the Volta much River scattering. Delta with Notably, the assoc sinceiate wesediment neglected load the for potentialthe period sediment from 1972 source to 2018 from (Figure the erosion 7) show of theed much lower scattering reach of the. Notably, river channel since we as aneglected result of thethe dampotential [36,37 sediment], the weak source correlation from the may erosion be partly of the due lower to the reach uncertainty of the river associated channel with as a the result estimated of the sedimentdam [36,37 load.], the Nevertheless, weak correlation the results may be from partly this due study to the suggested uncertainty the propositionassociated with from the the es previoustimated studiessediment [18 load.,39], Nevertheless, i.e., the dam constructionthe results from as the this major study influencing suggested the factor proposition of shoreline from change the previous of the Voltastudies River [18, Delta39], i.e., may the not dam hold construction at the current as stage. the major After influencing the construction factor of of the shoreline Akosombo change Dam, of there the couldVolta beRiver an initialDelta reductionmay not hold in sediment at the current load and stage subsequent. After the erosion construction of the shoreline of the Akosombo of the Volta RiverDam, there could be an initial reduction in sediment load and subsequent erosion of the shoreline of the Volta River Delta [18,39]. However, this lasted for a certain period until the delta reached a quasi- equilibrium state on the altered sediment supply. Besides, the attaining of the quasi-equilibrium state was presumably related to the rapid shoreline orientation changes induced by the high regional alongshore sediment transport [39], the beach nourishment, and the sea defense projects along the coast [20]. With a stable sediment load after the construction of the dams (Figure 3b), the delta was expected to remain in the quasi-equilibrium state unless other changes arise in the future.

Water 2020, 12, 646 10 of 12

Delta [18,39]. However, this lasted for a certain period until the delta reached a quasi-equilibrium state on the altered sediment supply. Besides, the attaining of the quasi-equilibrium state was presumably related to the rapid shoreline orientation changes induced by the high regional alongshore sediment transport [39], the beach nourishment, and the sea defense projects along the coast [20]. With a stable sediment load after the construction of the dams (Figure3b), the delta was expected to remain in the

Waterquasi-equilibrium 2020, 7, x FOR PEER state REVIEW unless other changes arise in the future. 10of12

) 30 -8 2 y =3.044×10 x -1.302 20 R2 =0.00493 10

-10

-20

Delta areal change (km -30

-40 7 -50 ×10 0 2 4 6 8 10 12 14 16 18 20 22 Cumulative sediment load (m3)

Figure 7. Relationship between the cumulative sediment load and areal change of the Volta River Delta. Figure 7. Relationship between the cumulative sediment load and areal change of the Volta River 4. ConclusionsDelta. The effects of the dam regulations on the hydrological alterations, including the inter- and 4. Conclusions intra-annual variations and the subsequent influence on the morphological evolution of the Volta RiverThe Delta, effects especially of the dam the regulations shoreline and on the delta hydrological areal changes alterations were analyzed, including in thisthe inter study.- and The intra river- annualdischarge variations and sediment and the load subsequent decreased influence by 23.23% on andthe 92.32%,morphological respectively, evolution after of dam the construction.Volta River DTheelta inter-, especially and intra-annual the shoreline variations and delta of river area dischargel changes andwere sediment analyzed load in are this more study. significant The river in dischargethe pre-dam and construction sediment load period decrease thand theby 23.23% post-dam and construction 92.32%, respectively, period as after expected. dam construction. The changes Thein shoreline inter- and and intra delta-annual area variations of the Volta of Riverriver discharge Delta from and 1972 sedimen to 2018t load are relativelyare more significant small, with in a themaximum pre-dam progradation-erosion construction period than rate the of 0.5%.post-dam The construction relationship period between as theexpected. variation The ofchanges the delta in shorelinearea and the and cumulative delta area sediment of the Volta load R isiver insignificant. Delta from It 1972 is reported to 2018 in are the relatively literature small that the, with Volta a maximumRiver Delta progradation underwent severe-erosion erosion rate of before 0.5%. theThe 1970s relationship due to thebetween reduced the sediment variation supply. of the delta However, area andthe presentthe cumulative results implicatesediment thatload the is insignificant. delta has approached It is reported a quasi-equilibrium in the literature statethat t sincehe Volta the R 1970s.iver DFurtherelta underwent analyses severe on the erosion land use before/cover the changes, 1970s due climate to the change, reduced morphodynamic sediment suppl adjustmenty. However, of the the presentdownstream results river implicate channel that on the the sedimentdelta has load approached to the Volta a quasi River-equilibrium Delta that is neglectedstate since in the this 1970s. study Furtherare recommended analyses on for the future land use/cover studies. Our changes, results climate provide change, references morphodynamic for the future adjustment development of andthe downstreamrestoration of river the Voltachannel River on the Delta. sediment load to the Volta River Delta that is neglected in this study are recommended for future studies. Our results provide references for the future development and restorationAuthor Contributions: of the VoltaConceptualization, River Delta. D.S. and M.A.; methodology, W.G. and M.A.; validation, D.S., W.G., and D.L.; formal analysis, W.G. and M.A.; investigation, M.A.; resources, M.A.; data curation, M.A.; writing—original Adraftuthor preparation, contributions: M.A.; Conceptualization, writing—review andD.S. editing, and M.A. D.L.,; m W.G.,ethod andology, D.S.; W.G. visualization, and M.A.; v M.A.;alidation, supervision, D.S., W.G. D.S., All authors have read and agreed to the published version of the manuscript. and D.L.; formal analysis, W.G. and M.A.; investigation, M.A.; resources, M.A.; data curation, M.A.; writing— originalFunding: draftThis preparation, work was supported M.A.; writing by the— Keyreview Project and of editing, the National D.L., W.G. Natural, and Science D.S.; Foundationvisualization, of M.A. China; s(grantupervision, 51639001), D.S. the Joint Funds of the National Natural Science Foundation of China (grant U1806217), and the Interdisciplinary Research Funds of Beijing Normal University. FAcknowledgments:unding: This work Towas the supported China Scholarship by the Key Council Project and of the Chinese National Government, Natural Science whose sponsorshipFoundation hasof China made (grantthis work 51639001), possible. the Joint To Water Funds Resources of the National Institute Natural of the Science Council Foundation for Scientific of China and Industrial (grant U1806217) Research,, and Accra, the InterdisciplinarGhana, and Voltay Research River Authority, Funds of Accra, Beijing Ghana, Normal for University. making data available for the project. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: To the China Scholarship Council and Chinese Government, whose sponsorship has made this work possible. To Water Resources Institute of the Council for Scientific and Industrial Research, Accra, Ghana, and Volta River Authority, Accra, Ghana, for making data available for the project.

Conflicts of Interest: The authors declare no conflict of interest.

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Water 2020, 12, 646 11 of 12

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